You can use "dpkg -l <package name>" or "dpkg -l | grep <package name>" (the "-l" is a small L) to see if it is installed. Installed indicates with a "ii" and uninstalled with "un" infront of package. No need for devel-su. If the output is empty then it doesn't exist in the repo and need to be installed through the browser.

To see dependancies you can use "apt-cache showpkg <package name>"